It is well known that a plasma containing free electrons and positive ions can be generated by heating vapor particles with RF energy. To do this, it is necessary to establish the proper conditions wherein the RF energy will effectively heat free electrons to ionize vapor particles, thus creating a plasma. One known way by which the transition from a vapor to a plasma can be effected is by radiating the vapor with a helicon wave in accordance with the so-called "helicon dispersion relation." Typically, this is accomplished in a cylindrical shaped vacuum chamber wherein a uniform magnetic field has been established and oriented substantially parallel to the longitudinal axis of the chamber. The mathematical expression for the helicon dispersion relation in this case is: EQU k.sub..parallel. k=.omega..sub.pe.sup.2.omega./.omega..sub.ce c.sup.2 =en.sub.e.mu..sub.o.omega./B
wherein
.omega..sub.ci &lt;.omega.&lt;.omega..sub.pe, .omega..sub.ce PA1 k is the magnitude of the wave vector for the helicon wave; PA1 k.sub..parallel. is a component of the wave vector that is parallel to the magnetic field inside the chamber; PA1 e is the electron charge; PA1 .omega..sub.pe is the electron plasma frequency; PA1 .omega. is the angular frequency of the RF energy; PA1 .omega..sub.ce is the electron cyclotron frequency; PA1 .omega..sub.ci is the ion cyclotron frequency; PA1 c is the speed of light; PA1 n.sub.e is the electron density of the plasma in the chamber; PA1 .mu..sub.o is the permeability of free space; and PA1 B is the magnitude of the magnetic field in the chamber. PA1 Accordingly, as can be easily seen from the above expressions, k.sub..parallel. k is proportional to n.sub.e. Similarly, .omega. is also proportional to n.sub.e. It then follows that as the electron density of a plasma (n.sub.e) changes (e.g. during an initial start up operation) the helicon dispersion relation can be maintained to continue the production of plasma by making appropriate changes in either k.sub..parallel. k or .omega..
It is to be appreciated that, while the helicon dispersion relation set forth above may imply there is but a single k.sub..parallel., in reality, an antenna will generate many k.sub.81 components. Accordingly, the condition lends itself to a Fourier analysis. For purposes of this disclosure, however, it is deemed sufficient to consider only an optimal k.sub..parallel. component and thereby forgo an in-depth Fourier analysis.
Heretofore, in accordance with the helicon dispersion relation, plasma generators have been designed and manufactured using the specific structural parameters that are required for a particular operating condition. This has meant that the antenna for generating the RF energy, as well as other components of the system, have had to be specifically designed. In this design process, additional factors, such as the type of plasma to be produced, the size of the chamber, and the desired density of the plasma inside the chamber, have had to be considered. Further, it has often been necessary to pre-ionize a portion of the vapor in order to initiate the operation. It happens, however, that it may be desirable in some operations to be able to sequentially generate different types of plasmas using the same chamber. Also, for operational efficiencies, it may be desirable to have the ability to initiate a plasma generating operation from a zero plasma condition.
With the above in mind, it is helpful to review the helicon dispersion relation and identify effective operational variables that can be manipulated to satisfy the relation. Specifically, from the relation it is to be noted that: EQU k.sub..parallel. k=en.sub.e.mu..sub.o.omega./B,
and that EQU .omega..sub.pe.sup.2.omega./.omega..sub.ce c.sup.2 =en.sub.e.mu..sub.o.omega./B.
In light of the above, it is an object of the present invention to provide a system and method for initiating and maintaining plasma production in a chamber which is operationally robust and effective over a range of electron densities. Another object of the present invention is to provide a system and method for initiating and maintaining plasma production in a chamber which is effective for producing different types of plasmas in successive operations using the same plasma chamber. Still another object of the present invention is to provide a system and method for initiating and maintaining plasma production in a chamber that is simple to operate, relatively easy to manufacture and comparatively cost effective.